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Short: the IJN trained a lot for night battles and didn’t cared about casualties and damage that can come with it. The USN considered it for to dangerous.

Pretty much this. Plus the IJN were involved in combat, or preparing for it, long before the USN began any type of serious training. Lot of the US military were in peacetime cost-cutting mode following WWI. Typically training, especially live fire training, gets cut out of the budget pretty early.

There is more to an optic than just how many times it enlarges the picture. There is also the diameter of the objective.

This diameter also indicates how well your optic will serve you in darker scenarios.

There's a rule of thumb for binoculars, let's take two glasses as examples. A 8x56 (A) and an 8x40 (B). Both will enlarge the picture eight times, but latter has a smaller objective.

Now we make the calculation for both glasses.

(A): (56/8)^2=49
(B): (40/8)^2=25

So, as the rule of thumb says, the first binocular will be a lot better in the dawn/darkness.

Yep the diameter helps you collecting light but of you have no idea what to look for it will not help you. There are two good books that show the Guadalcanal nightbattles from both sides: Neptune‘s Inferno for the USN and Japanese Destroyer Captain for the IJN. The second book shows also how much the Japanese underestimated radar in the later years. Same as the German navy. While they know that the British had radar they belived it was inferior.

Interestingly enough, the RN trained for night CV ops right from the get-go.

They got a few years more of naval tradition and experience under their belt. I blame this. The IJN also know that they have to fight against an enemy that will be superior in numbers. So they dedicated their strategy around decisive battle and reducing the numbers of the enemies ships with torpedos. Hence this explains ships like the Shimakaza.